This invention is based on and claims priority to Japanese Patent Application No. 2000-145987, filed May 18, 2000, and No. 2000-145988, filed May 18, 2000, the entire contents of which are hereby expressly incorporated by reference.
1. Field of the Invention
This invention relates to an exhaust system for an outboard motor, and more particularly to an improved exhaust system that has a main exhaust passage and an idle exhaust passage branched from the main exhaust passage.
2. Description of Related Art
An outboard motor typically comprises a power head including an internal combustion engine and a housing unit depending from the power head. The outboard motor typically employs an exhaust system that includes a main exhaust passage and an idle exhaust passage.
The main exhaust passage discharges exhaust gases that are produced at engine speeds above idle to the body of water surrounding the outboard motor through, for example, an exhaust pipe, an expansion chamber, and then through a submerged discharge port formed within the hub of a propeller. Under normal running conditions, when the engine speed is above idle, the exhaust pressure exceeds the back pressure caused by the body of water. The exhaust gases thus exit through the main exhaust passage. During idle, however, the exhaust pressure is less than the back pressure. Thus, only minimal amounts, if any, of the exhaust gases produced under idle engine speeds exit through the submerged port. Rather, substantially all of the exhaust gases produced during idle are discharged through an idle exhaust passage, through an idle port defined on the housing unit above the waterline.
Typically, the idle exhaust passage is branched from the main exhaust passage. Because of this arrangement, some exhaust gases tend to flow through the idle exhaust passage at engine speeds above idle and cause problems. One problem is that the exhaust flow can deposit carbons and/or lead components contained in the exhaust gases at a port of the idle exhaust passage. The deposits can accumulate sufficiently to close or narrow the passage port and prevent the idle exhaust gases from entering the idle exhaust passage smoothly.
A need therefore exists for an improved exhaust system for an outboard motor that can reduce the accumulation of deposits at a port of an idle exhaust passage.
Another problem of the arrangement is that some of the noise from exhaust produced at engine speeds above idle passes through the idle exhaust passage. Such exhaust carries more energy than exhaust generated during idle speed operation. Thus, exhaust noise generated by conventional exhaust systems can be louder than desired.
Another need thus exists for an improved exhaust system for an outboard motor that can attenuate exhaust noise generated during normal running conditions of the engine.
A further need exists for an improved exhaust system for an outboard motor that attenuates noise from the idle exhaust passage and does not excessively increase production costs.
In accordance with one aspect of the present invention, an outboard motor comprises an internal combustion engine. An intermediate member supports the engine. A housing unit depends from the intermediate member. An exhaust system guides exhaust gases from the engine. The exhaust system includes a main exhaust passage and an idle exhaust passage. The main exhaust passage discharges the exhaust gases produced at engine speeds above idle to the body of water in which the outboard motor operates, through the housing unit. The idle exhaust passage discharges the exhaust gases produced during idle engine speeds to the atmosphere. The intermediate member defines a portion of the main exhaust passage and a portion of the idle exhaust passage. The intermediate member also forms a recessed area in the main exhaust passage portion. The idle exhaust passage communicates with the main exhaust passage portion at the recessed area.
In accordance with another aspect of the present invention, an outboard motor comprises an internal combustion engine. A housing unit is disposed below the engine. An exhaust system guides exhaust gases from the engine. The exhaust system includes a main exhaust passage and an idle exhaust passage. The main exhaust passage discharges exhaust gases produced at engine speeds above idle to the body of water through the housing unit. The idle exhaust passage discharges exhaust gases produced during idle engine speeds to the atmosphere. An exhaust guide member defines a portion of the main exhaust passage. The exhaust guide member forms a recessed area in the main exhaust passage portion. An exhaust conduit communicates with the main exhaust passage portion to form a further portion of the main exhaust passage downstream from the main exhaust portion defined in the exhaust guide member. The idle exhaust passage is branched from the main exhaust passage portion at the recessed area and in proximity to the exhaust conduit.
In accordance with a further aspect of the present invention, an outboard motor comprises an internal combustion engine. A housing unit is disposed below the engine. An exhaust system guides exhaust gases from the engine. The exhaust system includes a main exhaust passage and an idle exhaust passage. The main exhaust passage discharges the exhaust gases produced at engine speeds above idle to the body of water through the housing unit. The idle exhaust passage discharges exhaust gases produced during idle engine speeds to the atmosphere. An exhaust guide member defines a portion of the main exhaust passage. The idle exhaust passage is branched from the main exhaust passage portion. The idle exhaust passage extends at an acute angle relative to a direction of exhaust gas flow in the main exhaust passage.
In accordance with a yet another aspect of the present invention, an outboard motor comprises an internal combustion engine. A housing unit is disposed below the engine. An exhaust system guides exhaust gases from the engine. The exhaust system includes a first exhaust passage and a second exhaust passage. The first exhaust passage discharges exhaust generated during relatively a high engine speeds to the body of water through the housing unit. The second exhaust passage discharges exhaust gases generated during relatively low engine speeds, to the atmosphere. A support member is arranged to support the engine. The support member defines a portion of the first exhaust passage. The support member forms a recessed area in the first exhaust passage portion. The second exhaust passage is branched off from the first exhaust passage portion at the recessed area and at a location distal from the engine.
In accordance with a yet further aspect of the present invention, a method is provided for forming an exhaust guide member of an outboard motor. The exhaust guide member defines a main exhaust passage and a secondary exhaust passage branched from the main exhaust passage within the exhaust guide member. The method comprises placing a first mold in a cast frame of the exhaust guide member, placing a second mold in the cast frame, casting the exhaust guide member, drafting the first mold from the cast frame, drafting the second mold from the cast frame in a direction opposed to a direction in which the first mold is drafted, and boring an aperture in a recessed area formed by one of the first or second mold.
Further aspects, features and advantages of this invention will become apparent from the detailed description of the preferred embodiment which follows.